Abstract
A main purpose of the current investigation was to determine if Fitts’ index of difficulty [log2(2A/W)] could be taken as an index of subjective difficulty in prospective action. In two experiments, participants viewed 12 target displays with values of log2(2A/W) (prescribed difficulty) ranging between 1.0 and 6.5 bits. Following each 15 s trial, participants provided magnitude estimates reflecting the difficulty that someone else would experience if they actually had to make targeted movements during the trial. The prospective difficulty estimates were always made in the absence of movement. In Experiment 1, target displays were presented to participants on their own video monitor, while in Experiment 2, all participants concurrently viewed scaled-up target displays projected onto a large screen. There were three main findings: First, in both experiments, the prospective-prescribed relation was strong and positive. This finding warrants two conclusions: Fitts’ index of difficulty can be taken as an index of subjective difficulty in prospective action, and subjective estimates of performance difficulty result from the monitoring of feedforward control signals generated in the absence of movement-related feedback. Second, even with the large differences in the target display scale of Experiments 1 and 2, difficulty estimates were equivalent at common prescribed difficulty levels. In other words, the results of Experiment 1 were successfully replicated in Experiment 2. This finding demonstrates the generality of the prospective-prescribed relation. Third, nonlinearities in the prospective-prescribed relation resembled those seen in functions describing the increases in movement time that accompany increases in prescribed difficulty (Fitts’ law). This observation suggests that the prospective difficulty estimates were based on the value of a temporal parameter in an implicit mental simulation of the action.
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Notes
The original purpose of instructing participants to make prospective difficulty judgments from a third-person perspective was to maximize the implicit nature of the task. We thought that if participants were asked to make judgments from a first-person perspective, then this would increase the probability of them engaging in explicit action simulation. However, because prospective action judgment is known to engage implicit action simulation (e.g. Johnson 2000), which in all likelihood involves a first-person perspective, and because we did not provide a specific third-person model, it seems probable that our participants defaulted to a first-person perspective. In contrast, in other studies on perspective taking in imagined action, the experimenter has served as an explicit third-person model for participants’ action simulations (Ruby and Decety 2001; Sirigu and Duhamel 2001). In those studies, as compared with the current study, it would have been easier for participants to form a third-person, experimenter image that was distinct from the participants’ first-person image.
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Slifkin, A.B., Grilli, S.M. Aiming for the future: prospective action difficulty, prescribed difficulty, and Fitts’ law. Exp Brain Res 174, 746–753 (2006). https://doi.org/10.1007/s00221-006-0518-3
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DOI: https://doi.org/10.1007/s00221-006-0518-3